1. Technical Field
The present invention relates to digitizer systems and to parts thereof. The invention has particular relevance to inductive digitizers for use with portable electronic equipment such as a tablet PC, PDA or mobile telephone.
2. Related Art
U.S. Pat. No. 4,878,553 describes an inductive digitizer system having loop coils arrayed in X and Y directions. The digitizer includes an excitation circuit for applying an energizing signal to a selected loop coil for energizing a resonant stylus. Once energized, the resonant stylus emits a response magnetic field which couples back into the loop coils. Processing electronics of the digitizer then process the signals from the loop coils to determine a measurement of the X-Y position of the stylus over the digitizer.
The system described in U.S. Pat. No. 4,878,553 has a number of problems. One problem is that there are a large number of coil crossovers required at the edge of the sensor board. For best accuracy and resolution, the coils should approach the edge of the sensor board as closely as possible. This means that when the loop coils are formed from conductors on printed circuit boards, the PCB geometries (line widths, gaps and via sizes) must be small, yielding an expensive product. The alternative is for these geometries to be large, at the expense of accuracy. Further, in view of the large number of crossovers, there are a relatively large number of connections between the layers of the printed circuit board. Since each connection has a finite chance of failure at manufacture, the manufacturing process must yield high reliability for each such connection, thereby again increasing manufacturing costs.
A further problem with the system described in U.S. Pat. No. 4,878,553 is that typically it draws a relatively high current from its supply, for a given noise level, due to inefficiencies in coil layout and the way in which the coils are energized to energize the resonant stylus.
An aim of the present invention is to address at least one of these problems and to provide an alternative digitizer system or to provide alternative components for use in such a digitizer system.
In the exemplary embodiment described below, a new digitizing system is described having a new arrangement of windings for energizing and sensing the position of a resonant stylus above a working surface thereof. The digitizing system also includes novel excitation and multiplexing circuitry that is used to select a winding to be energized for energizing the resonant stylus. The digitizing system also includes novel processing circuitry that is used to control the operation of the digitizer so that it can operate with different types of stylus.